(a) Field of the Invention
The present invention relates to a display device and a driving method thereof.
(b) Description of Related Art
A typical liquid crystal display (LCD) device includes a lower panel and an upper panel and a liquid crystal (LC) layer with dielectric anisotropy disposed between the lower and upper panels. The lower and upper panels are provided with pixel electrodes and a common electrode. The pixel electrodes are arranged in a matrix and each pixel electrode is connected to a switching element Q such as a thin film transistor (TFT). Each row of pixel electrodes is sequentially supplied with data voltages. The common electrode covers an entire surface of the upper panel and is supplied with a common voltage. The pixel electrode, the common electrode and the LC layer therebetween form an LC capacitor, and the LC capacitor is a basic unit comprising each pixel together with the switching element Q.
The LCD device displays desired images by adjusting a strength of an electric field applied to the LC layer to control a transmittance of light passing through the lower and upper panels. In order to prevent deterioration of the LC layer due to applying one directional electric fields to the LC layer, a polarity of the data voltages is reversed with respect to the common voltage for each frame, each pixel row or each pixel.
Recently, products to provide such LCD devices with a photosensor have been developed. When, for example, a hand or a touch pen touches a screen of an LCD device, the photosensor provides a response to a variation of light according to a position of the hand or the touch pen. The LCD device determines touch information such as presence or absence of a touch and a position of the touch for transmittance to an external device. The external device transmits image signals responsive to the touch information. The photosensor is formed by a separate touch panel in the LCD device. However, such a separate touch panel increases a thickness and weight of the LCD device and makes it hard to display detailed characters or pictures.
Therefore, a technique of forming a photosensor in a pixel displaying images has been developed. However, such a photosensor causes many errors in photo sensing responsive to touch since an output characteristic of the photosensor is varied depending on ambient circumstances, i.e., an intensity of external light, a luminance of a backlight, temperature, and so on. Accordingly, the LCD device may perceive no touch although a touch has occurred or may perceive a touch although no touch has occurred.